There are many good oxidation and corrosion resistant coatings used in industry for various applications and for use in various environments. Articles composed of iron-, cobalt-, or nickel-based superalloys have been developed for use in applications, such as aerospace applications, and for use as blades, vanes, seals and other components utilized in gas turbine engines. In these applications, it is important that the articles have sufficient protection against undue oxidation and sulfidation since such corrosion can affect the useful life of the article resulting in reduced performance and possible safety problems. Although various superalloys have a high degree of corrosion resistance, such resistance decreases when the superalloys are operated in or exposed to high temperature environments.
To increase the useful life of components made of alloys and superalloys, various coatings have been developed. Aluminide coatings were initially used to provide a corrosion resistant outer layer but such layer was observed to crack when subjected to mechanically or thermally induced strain. Another class of coatings developed was the MCrAlY overlay coatings where M represents a transition metal element such as iron, cobalt or nickel. The coatings have been found to be more effective than the aluminide coatings in extending the useful life of alloy components in high temperature environments.
Modern gas turbine engines operate in a high temperature environment in excess of 2000.degree. F. in which hot gases are expanded across rows of turbine blades. These turbine blades are typically nickel base alloys chosen for excellent high temperature creep and thermal fatigue resistance. In general, the design of the blade alloy sacrifices resistance to oxidation and hot corrosion in order to achieve the optimized mechanical properties. Therefore, the blade is coated with a thin layer of material designed to provide only high resistance to oxidation or hot corrosion, with little regard to mechanical properties of the coating. This thin coating, typically 3 to 8 mils thick, is generally applied by argon shrouded plasma spraying, plasma spraying in a vacuum chamber, or by physical vapor deposition methods.
In the field of gas turbine engines, designers continually look toward raising the operating temperature of the engine to increase efficiency. In turn, higher temperatures act to reduce the life of the current coatings on the turbine blades and vanes. Components of a gas turbine engine can also be subjected to hot corrosion. This can occur when there is salt ingested into the engine via the intake air, or when the fuel has even low levels of sulfur concentration, or both. The attack of bare blades or even coatings on blades can be very rapid in hot corrosion, where the sulfur and salt can form liquid compounds on the surface that are able to dissolve the otherwise protective oxide scale on the substrate. This hot corrosion mechanism is most aggressive when the blade temperature falls between the temperatures where the complex salt-sulfate compounds melt and the temperature where the compounds evaporate. In the intermediate temperature range a liquid film of the corrodant can exist on the surface of the substrate and be very deleterious. Even in engines that generally run at high temperatures, above the evaporation temperature of such liquid corrodants, there may be conditions where the components pass through the lower temperature regime, such as during reduced power operation or at idle waiting for take-off in an aero engine. If the corrodants are present in the air or fuel, they can enhance the rate of attack during these periods.
In operation, the turbine blade experiences a range of temperatures as the power demand is raised or lowered. The blade also experiences a range of axial stresses as the rotation speed of the blade is increased or decreased. Of course, both the temperature change and the stress change happen concurrently to the rotating blade. One mode is when temperature and tensile stress both increase together as the demand for power is increased, and they both decrease together as power is reduced. If the blade temperature were plotted on the abcissa and the stress on the ordinate of an x-y graph, the above mode would look like a single upward sloping line in the positive stress and temperature quadrant. It is possible when temperature changes quickly or the surface of the blade is heated or cooled faster than the core of the blade, that the graph of the total power cycle is not the same simple curve for heating and cooling. Rather the heating and cooling legs of the stress-temperature graph may be different, and the total cycle looks like an open loop. This is an indication of hysteresis in the system between stress and temperature.
If now a thin coating is applied to the blade surface, and the coating has a different thermal expansion rate than the blade alloy, the situation becomes more complicated. One can envision separate stress-temperature graphs for the coating and the blade alloy for the same power cycle. In many cases, the thermal expansion rate of MCrAlY coatings is greater than that of typical nickel base blade alloys. Considering the stress-temperature graph of the coating, there would be two contributions to its stress state. One would be the radial tensile stress due to increasing the rotational speed of the blade. The stress in the coating would be the same as in the underlying blade due to this effect. In addition, since the coating is assumed to expand faster than the blade alloy, it wants to become longer than the blade but is well bonded to the substrate so it is constrained and a compressive stress develops in the coating. The total coating stress is the sum of the two contributions. The heating leg of the coating graph will thus have less tensile stress than the blade because of the compression component, so its curve would increasingly fall below the simple line assumed for the bare blade. If all the high temperature stress was able to be stored in the coating, when it experienced the cooling leg of the cycle it would trace back along the heating leg for the coating. However, most MCrAlY coatings are weak at high temperatures in comparison to blade alloys, and some of the stress in the coating would be reduced due to annealing or creep. In that case, when the cooling leg of the cycle occurs, the coating stress will end up at a lower value at the final low temperature than what it was at the start. This is due to the stress relaxation effect of the weak coating at the high temperature. Depending on the relative contributions of the stress due to blade spinning compared to the differential thermal expansion stress effects, and the number of cycles of heating and cooling, the coating could become increasingly more in compression. A mechanism such as described here could be responsible for the observation that some coatings become buckled and cracked after many cycles.
A further current problem with conventional MCrAlY coatings on superalloy substrates is interdiffusion of coating elements into the substrate and substrate elements into the coating after long times of high temperature exposure. The loss of coating aluminum to the substrate is noticed by an aluminide depletion layer in the coating. Certain substrate elements like titanium have been found to diffuse through the MCrAlY coating to the external surface oxide scale and to make said oxide scale less protective. It would be desirable to modify current MCrAlY coatings to reduce this interdiffusion effect.
Although MCrAlY has overall been a successful class of coatings having good oxidation and corrosion resistance for superalloys, improvements have been made to the MCrAlY coatings.
U.S. Pat. No. 3,676,085 discloses that the oxidation-erosion and sulfidation resistance of the nickel- and cobalt-based superalloys is markedly improved through the use of a coating consisting of cobalt, chromium, aluminum and an active metal such as yttrium, particularly at the composition, by weight, of 15-40 percent chromium, 10-25 percent aluminum, 0.01-5 percent yttrium, balance cobalt.
U.S. Pat. No. 3,754,903 discloses a coating alloy for the gas turbine engine super-alloys which consists primarily of nickel, aluminum and a reactive metal such as yttrium, particularly at the composition, by weight, 14-30 percent aluminum, 0.01-0.5 percent reactive metal balance nickel. A preferred embodiment also includes 15-45 weight percent chromium.
U.S. Pat. No. 3,928,026 discloses a highly ductile coating for the nickel- and cobalt-base superalloys having long term elevated temperature oxidation-erosion and sulfidation resistance and diffusional stability which coating consists essentially of, by weight, 11-48% Co, 10-40% Cr, 9-15% Al, 0.1-1.0% reactive metal selected from the group consisting of yttrium, scandium, thorium, lanthanum and the other rare elements, balance essentially Ni, the nickel content being at least about 15%.
U.S. Pat. No. 3,993,454 discloses coatings which are particularly suited for the protection of nickel and cobalt superalloy articles at elevated temperatures. The protective nature of the coatings is due to the formation of an alumina layer on the surface of the coating which serves to reduce oxidation/corrosion. The coatings contain aluminum, chromium, and one metal chosen from the group consisting of nickel and cobalt or mixtures thereof. The coatings further contain a small controlled percentage of hafnium which serves to greatly improve the adherence and durability of the protective alumina film on the surface of the coating. U.S. Pat. No. 4,585,481 discloses a similar coating except that yttrium and hafnium are used together along with silicon.
U.S. Pat. No. 3,918,139 discloses a nickel, cobalt and nickel-cobalt alloy coating composition having improved hot corrosion resistance. In particular, an improved MCrAlY type alloy coating composition consists essentially of, by weight, approximately 8-30 percent chromium, 5-15 percent aluminum, up to 1 percent reactive metal selected from the group consisting of yttrium, scandium, thorium and the other rare earth elements and 3-12 percent of a noble metal selected from the group consisting of platinum or rhodium, the balance being selected from the group consisting of nickel, cobalt and nickelcobalt.
U.S. Pat. No. 4,677,034 discloses an MCrAlY coating in which silicon is added. U.S. Pat. No. 4,943,487 disclosed a NiCrAlY or NiCoCrAlY coating to which tantalum is added. U.S. Pat. No. 4,743,514 discloses a coating for protecting the surfaces of gas turbine components such as single crystal turbine blades and vanes, wherein the coating has a composition (in weight percent) consisting essentially of chromium, 15-35; aluminum, 8-20; tantalum, 0-10; tantalum plus niobium, 0-10; silicon, 0.1-1.5; hafnium, 0.1-1.5; yttrium, 0-1; cobalt, 0-10; and nickel, balance totalling 100 percent. A preferred coating, which is particularly desirable for use with single-crystal turbine blades and vanes, has a composition consisting essentially of chromium, 17-23; aluminum, 10-13; tantalum plus niobium, 3-8; silicon, 0.1-1.5; hafnium, 0.1-1.5; yttrium, 0-0.8; cobalt, 0-trace; and nickel, balance totalling 100 percent. A process for preparing the coated component is also described.
U.S. Pat. No. 4,615,864 disclosed coatings for iron-, nickel- and cobalt-base superalloys. The coatings are applied in order to provide good oxidation and/or sulfidation and thermal fatigue resistance for the substrates to which the coatings are applied. The coatings consist essentially of, by weight, 10 to 50% chromium, 3 to 15% aluminum, 0.1 to 10% manganese, up to 8% tantalum, up to 5% tungsten, up to 5% reactive metal from the group consisting of lanthanum, yttrium and other rare earth elements, up to 5 percent of rare earth and/or refractory metal oxide particles, up to 12% silicon, up to 10% hafnium, and the balance selected from the group consisting of nickel, cobalt and iron, and combinations thereof. Additions of titanium up to 5% and noble metals such as platinum up to 15% are also contemplated.
U.S. Pat. No. 4,101,713 discloses a coating made from mechanically alloyed MCrAl with a dispersoid of Al.sub.2 O.sub.3, ThO.sub.2 or Y.sub.2 O.sub.3.
It is an object of the present invention to provide an improved coating having good high temperature oxidation resistance characteristics.
It is another object of the present invention to provide a coating for substrates that are intended to operate in high temperature oxidizing and sulfidizing environments.
It is another object of this invention to provide a coating for superalloy substrates that will have a thermal expansion rate that is similar to that of the substrates and will have a greater high temperature strength so that it will resist stress relaxation.
It is another object of the present invention to improve the diffusional stability of the coating toward nickel and cobalt base substrates.